Auto merge of #86695 - sexxi-goose:closure_size, r=nikomatsakis

Auto merge of #86695 - sexxi-goose:closure_size, r=nikomatsakis

Introduce -Zprofile-closures to evaluate the impact of 2229

This creates a CSV with name "closure_profile_XXXXX.csv", where the
variable part is the process id of the compiler.

To profile a cargo project you can run one of the following depending on
if you're compiling a library or a binary:

```
cargo +nightly rustc --lib -- -Zprofile-closures
cargo +nightly rustc --bin {binary_name} -- -Zprofile-closures
```

r? `@nikomatsakis`